For the purpose of improving crack resistances of roll materials in steel rolling mill, the effects of carbon content (C) and heat treatment on fracture toughness and fatigue crack propagation were studied using 17mass%Cr cast iron. The main results obtained from this study are summarized as follows ; Fracture toughness (KIC) decreases with increasing C content, but it rises to the maximum value in the eutectic composition, and drops conspicuously when the C content exceeds the eutectic value. The range of threshold stress intensity factor for fatigue crack propagation (ΔKIth) decreases with rising C content in the same manner as the variation of KIC. It was found that Paris-Erdogan equation, da/dN=C (ΔKI)m held well between the crack propagation rate and range of stress intensity factor, and that there was a constant relation of C=A/Bm. Hardening treatment deteriorates ΔKIth because of martensite formation in the matrix. In as-cast specimens with high ΔKIth values, constant m decreases with increasing ΔKIth except for eutectic alloy, and in hardened specimens with low ΔKIth value, m increases with an increase in the ΔKIth value.
